This invention generally relates to the fabrication of a strongly linked array of high-temperature superconductors. Using this process, strongly linked superconductors with complex geometries and shapes can be fabricated.
This process relies on a joining technique between two individual strongly linked superconductors. Specifically, this invention relates to fabrication of single domains of YBa2Cu3Ox or YBa2Cu3Ox with the addition of Y2BaCuO5 and/or other secondary phases such as Pt/PtO2, CeO2, SnO2, Ag, Y2O3 and other rare earth oxides, by using a top-seeded, melt processing technique. Beginning with a single crystal seed such as Nd1+yBa2xe2x88x92xCu3Oy or SmBa2Cu3Ox crystals, a melt-textured YBCO domain with crystallographic orientation nearly similar to that of the seed crystal can be fabricated. The samples are next machined to desired geometrical shapes. A bonding material is then applied to the ac plane. Low solidification or recrystalization point, similar crystal structure to that of YBa2Cu3Ox, and capability of growing epitaxially on YBCO domains are critical parameters of the bonding material. The pre-bonded samples are then heat treated such that the bonding material grows epitaxially on the ac plane of YBCO domains resulting in a strongly linked joint. A small amount of pressure may be used to keep neighboring domains together.
This invention also relates to fabrication of a strongly linked array of YBCO for various applications including, for example, frictionless bearings, cylinders for shields and fault current limiters, and long-length superconductors for current leads and bus bars.
One of the major problems inhibiting widespread application of high-temperature superconductors is their poor current-carrying capability (critical current density, xe2x80x9cJcxe2x80x9d). In particular the Jc is limited by the weak links at the grain boundaries and by poor flux pinning properties. Researchers have had some success in reducing the weak links by various texturing techniques. The most successful and popular techniques for overcoming the weak link problem have been the melt-texturing methods in the YBCO system and mechanical alignment or deformation induced texturing accompanied by partial melting in the BSCCO system.
Melt-processing techniques offer an attractive way to fabricate strongly linked YBCO. Moreover, defects such as fine Y2BaCuO5 inclusions and other structural defects (i.e., stacking faults and dislocations) can be incorporated during processing and can further enhance the current-carrying capability in these materials. In a typical melt-processing method, YBCO is heated above its peritectic point where it melts incongruently into Y2BaCuO5 and a Ba- and Cu-rich liquid. The semi-solid melt is then cooled slowly (at around xcx9c1xc2x0 C./h about i.e. from 0.25xc2x0 C./h to about 5xc2x0 C./h) to obtain aligned grains of YBCO. The grains are aligned, however, only in small regions called domains. The superconductor is mostly strongly-linked within the domain by mostly strongly as used herein this might be from 70%-90%, preferably from 75% to 85% linked, wherein linking refers to conductive flow between domains at high magnetic fields. The domains are separated by large-angle grain boundaries which generally act as weak links, thereby reducing the current-carrying capability in the presence of magnetic fields. The size of the strongly linked regions can be enhanced by increasing the domain size in the melt-processed YBCO. One way to increase the domain size is to use a directional solidification method with a controlled temperature gradient and by using a seed to initiate the grain growth. The use of a seed not only ensures a single nucleation site, but also permits controlled orientations of the grains.
Considerable progress in increasing the size of single-domain YBCO has been achieved through seeding methods which have the capability to control the crystal orientation of the xe2x80x9cas-grownxe2x80x9d single domains. Even with the advance in seeding methods, the diameter of the single domain is typically limited to roughly 50 mm. This size limitation is believed to be in part due to the high supercooling along the direction of growth which often leads to disorderly production of nuclei at the later stage of crystal growth resulting in multi-domained HTS (high-temperature superconductors). The limitation is also believed to be due to the lower temperature at the edges of the sample where grain nucleation can occur resulting in multiple impinging domains. Only under special conditions such as a composition gradient and/or a sizable temperature gradient can a large, single-domained YBCO be obtained. For example, using combined radial and vertical composition gradients, a 50-mm diameter sample with a trapped field of 1.27 T at 77K has been reported. This impressive value, however, indicates that a Jc of only 1xc3x97104 A/cm2 is sustained by the sample. This Jc value is considerably smaller than that of small-domain specimens (3xc3x97104 A/cm2 at 77K and 1 T), and indicates that weak links are still present in the apparent single-domained HTS. The decreasing quality of large, single-domained HTS has been confirmed by X-ray rocking curve analyses where the c-axis spread in textured YBCO, near the seed, is found to be about 1.5xc2x0 to 2xc2x0 and increases to 3xc2x0 to 15xc2x0 near the edge of a 30-mm diameter sample. As a result of this deterioration in crystal perfection, the rate of increase in trapped field decreases drastically when the sample size exceeds roughly 30 mm. Consequently, there is a need for the development of a processing technique that can produce large single-domain HTS with high Jc (i.e., of at least about 10,000 A/cm2 at 77K and without any weak links. Furthermore, fabrication of strongly linked complex shapes like rings and cylinders are difficult by seeding techniques.
This invention addresses a technique suitable for developing a strongly linked array of independent domains resulting in complex shapes desirable for various applications such as magnetic shields, fault current limiters, superconducting magnet bearings, current leads, and the like.